A wind powered generator typically consists of a turbine having a propeller and drive shaft supported on a mast for rotation about a horizontal axis. The drive shaft is connected to an electrical generator so that rotation of the turbine produces electrical power from the generator. One of the fundamental characteristics of a wind turbine is that its rotational speed varies according to the prevailing wind conditions. Those wind conditions in term may vary dramatically over a short time, and it is therefore necessary to consider control of the turbine over a wide range of conditions.
The rotational speed of the turbine must be limited to maintain efficiency and to avoid excessive mechanical forces that might be imposed upon the blades of the propeller in the event of over speeding.
Larger turbine installations have sophisticated control systems that adjust the pitch of the blades to maintain the speed below the set limit. However, such installations are relatively expensive and require significant maintenance and are therefore not suitable for lower powered installations.
Other systems rely upon mechanical braking, or the movement of the blades to a less efficient position relative to the wind to limit the power generated.
PCT Application No. 2010/032012 shows a hydraulic drive system for use in a wind turbine in which the hydraulic drive is used to drive a generator and the rotational speed of the rotor is controlled so as to provide a constant rotational speed of the generator. Such a drive system, however, is relatively complex and, because flow of the fluid is constantly adjusted to regulate the speed, the drive system is subject to continuous hydraulic losses that render the overall efficiency of the system lower than that of a direct mechanical drive to a generator.
It is therefore an object of the present invention to provide a wind turbine in which the above disadvantages are obviated or mitigated.